How long the battery of a GRETA holds depends on the usage and the environment. The discharge speed can thus always vary a little.
If GRETA is actively used, there are always new positions. The transmission of this data always consumes energy a little bit of energy. Of course this can be controlled to a certain extent over the selectable interval.
However, if GRETA is in an area where it is difficult to get a GPS position or network connection, the power consumption is slightly higher. This slight increase comes about because it takes longer to transfer data. You may also have noticed this with your mobile phone: If there is only a slow network connection, the battery runs out faster.